1. Field of the Invention
The invention relates to a carrier for receiving semiconductor wafers for the machining thereof in grinding, polishing and lapping machines, a method for coating a carrier, and also a method for the simultaneous double-side material-removing machining (lapping, grinding or polishing) of semiconductor wafers using such carriers.
2. Background Art
Electronics, microelectronics and microelectromechanics require as starting materials (substrates) semiconductor wafers with extreme requirements of global and local flatness, front-side-referenced local flatness (nanotopology), roughness, cleanliness and freedom from impurity atoms, in particular metals. Semiconductor wafers are wafers made of semiconductor materials such as compound semiconductors, for example, gallium arsenide, or elemental semiconductors such as principally silicon and occasionally germanium or else layer structures thereof. Layer structures are for example a device-carrying silicon upper layer on an insulating interlayer (“silicon on insulator”, SOI), or a lattice-strained silicon upper layer on a silicon/germanium interlayer with germanium proportion increasing toward the upper layer, on a silicon substrate (“strained silicon”, s-Si), or combinations of the two (“strained silicon on insulator”, sSOI). Semiconductor materials are preferably used in monocrystalline form for electronic components, and are preferably used in polycrystalline form for solar cells (photovoltaics).
In order to produce the semiconductor wafers in accordance with the prior art, a semiconductor ingot is produced which is firstly separated into thin wafers, usually by means of a multiwire saw (“multiwire slicing”, MWS). This is followed by one or more machining steps which can generally be classified into the following groups:    a) mechanical machining;    b) chemical machining;    c) chemomechanical machining;    d) if appropriate production of layer structures.A multiplicity of secondary steps such as edge machining, cleaning, sorting, measuring, thermal treatment, packaging, etc. are also used.
Mechanical machining steps in accordance with the prior art are lapping (simultaneous double-side lapping of a plurality of semiconductor wafers in a “batch”), single-side grinding of individual semiconductor wafers with single-side clamping of the workpieces (usually carried out as sequential double-side grinding; “single-side grinding”, SSG; “sequential SSG”) or simultaneous double-side grinding of individual semiconductor wafers between two grinding disks (simultaneous “double-disk grinding”, DDG).
Chemical machining comprises etching steps such as alkaline, acidic or combination etches.
Chemomechanical machining comprises polishing methods in which material removal is obtained by means of relative movement of semiconductor wafer and polishing cloth under pressure while supplying a polishing slurry (for example an alkaline silica sol). The prior art describes batch double-side polishing (DSP) and batch and individual wafer single-side polishing (mounting of the semiconductor wafers by means of vacuum, adhesive bonding or adhesion during the polishing machining on one side on a support).
For producing highly planar semiconductor wafers, particular importance is ascribed to those machining steps in which the semiconductor wafers are machined largely in a constrained-force-free manner in “free-floating” fashion without force-locking or positively locking clamping (“free-floating processing”, FFP). Undulations such as are produced for example by thermal drift or alternating load in MWS are eliminated particularly rapidly, by FFP with little loss of material.
FFP known in the prior art include lapping, DDG and DSP, where DDG will not be considered in the context of this invention due to different kinematics. A lapping method is disclosed e.g. in Feinwerktechnik & Messtechnik 90 (1982) 5, pp. 242-244, while a DSP method is described e.g. in Applied Optics 33 (1994) 7945.
DE 103 44 602 A1 discloses a further mechanical FFP method in which a plurality of semiconductor wafers lie in a respective cutout of one of a plurality of carriers that are caused to effect rotation by means of a ring-shaped outer and a ring-shaped inner drive ring, and are thereby held on a specific geometrical path and machined in material-removing fashion between two rotating working disks coated with bonded abrasive. This method is also termed “Planetary Pad Grinding” or simply PPG. The abrasive is composed of a film or “cloth” bonded to the working disks of the apparatus used, as disclosed in U.S. Pat. No. 6,007,407, for example.
Hard substances are used as the abrasive, e.g. diamond, silicon carbide (SiC), cubic boron nitride (CBN), silicon nitride (Si3N4), cerium dioxide (CeO2), zirconium dioxide (ZrO2), corundum/aluminum oxide/sapphire (Al2O3) and many other ceramics having grain sizes of less than 1 micrometer up to a few tens of micrometers. For the machining of silicon in particular, diamond is preferred, and furthermore also Al2O3, SiC and ZrO2. The diamond is incorporated as individual grains or bonded by means of a ceramic, metallic or synthetic resin primary bond to form conglomerates, into the ceramic, metal or synthetic resin matrix of the abrasive bodies.
DE 103 44 602 A1 discloses a method in which either a multiplicity of abrasive bodies containing bonded abrasive are bonded to the working disks or in which the abrasive is bonded in a layer or a “cloth” and cloths of this type are bonded to the working disk. The working layer may also be adfixed by means of vacuum, screwing, covering or by means of hook and loop fastening, or in electrostatic or magnetic fashion (see e.g. U.S. Pat. No. 6,019,672 A). Sometimes the working layers are embodied as cloths or laminated sheets (U.S. Pat. No. 6,096,107 A, U.S. Pat. No. 6,599,177 B2).
Sheets having structured surfaces are also known, comprising elevated regions that come into contact with the workpiece and recessed regions via which cooling lubricant can be supplied and abrasive slurry and spent grain can be discharged. An abrasive tool (abrasive cloth) structured in this way is disclosed by U.S. Pat. No. 6,007,407 A, for example. Here the abrasive cloth is self-adhesive on the rear side, which permits a simple change of the abrasive tool on the working disk.
Suitable apparatuses for carrying out the machining methods (lapping, DSP and PPG) appertaining to the invention essentially comprise a ring-shaped upper and lower working disk and a rolling apparatus comprising toothed rings arranged on the inner edge and on the outer edge of the ring-shaped working disks. Upper and lower working disks and inner and outer toothed rings are arranged concentrically and have collinear drive axes. The workpieces are introduced into thin guide cages which are toothed on the outside, so-called “carriers”, which are moved between the two working disks during machining by means of the rolling apparatus.
In the case of PPG, the working disk comprises, as mentioned above, a working layer with fixedly bonded abrasive. In the case of lapping, use is made of working disks, so-called lapping plates, composed of cast material, generally a steel casting, e.g. ductile gray cast iron. These contain in addition to iron and carbon a multiplicity of nonferrous metals in different concentrations. In the case of DSP, the working disks are covered with a polishing cloth, wherein the polishing cloth is composed for example of a thermoplastic or heat-curable polymer. A foamed plate or a felt or fiber substrate which is impregnated with a polymer is also suitable. In the case of lapping and DSP, lapping and polishing agents, respectively, are additionally supplied.
For lapping, oils, alcohols and glycols are known as carrier liquids for the lapping agent (abrasive substance slurry, abrasive substances), also called slurry. For DSP, aqueous polishing agents to which silica sol is applied are known, which are preferably alkaline and, if appropriate, contain further additives such as chemical buffer systems, surfactants, complexing agents, alcohols and silanols.
In the prior art, carriers are known which comprise, e.g., disks composed of a first hard, stiff material, e.g. steel, in particular high-grade steel, which are toothed on the outside appropriately to match the rolling apparatus, have holes in their surface for passage of the cooling lubricant, and one or more cutouts for receiving one or more semiconductor wafers, wherein the cutouts for receiving the semiconductor wafers are usually lined with a second, softer material.
These linings are introduced loosely into the cutouts (JP 57041164) or fixed in the latter (EP 0 197 214 A2). Fixing can be effected by adhesive bonding or positive locking, if appropriate with support by enlarged contact areas (corresponding polygons in cutout and lining) or else by anchoring by means of corresponding undercuts (“dovetail”) (EP 0 208 315 B1).
Materials known in the prior art for the lining are e.g. polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE) (EP 0 208 315 B1), and also polyamide (PA), polystyrene (PS) and polyvinylidene difluoride (PVDF).
Carriers are likewise known which are produced from only a single, sufficiently stiff material, e.g. a high-performance plastic or a plastic having a reinforcement made of e.g. glass, carbon or synthetic fibers (JP 2000127030 A2). U.S. Pat. No. 5,882,245 discloses carriers composed of polyether ether ketone (PEEK), polyaryl ether ketone (PAEK), polyetherimide (PEI), polyimide (PI), polyether sulfone (PES), polyamideimide (PAI), polyphenylene sulfide (PPS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), acetal homopolymer (POM-H), acetal copolymer (POM-C), liquid crystal polymer (LCP), and epoxy (EP). U.S. Pat. No. 5,882,245 also discloses carriers having applied protective coats of lacquer based on epoxy (EP), epoxy-acrylate mixture (EP/AC), polyurethane-acrylate mixture (PU/AC) or epoxy-acrylate-polyurethane (EP/AC/PU).
For application in the case of lapping, usually a single-layered steel or high-grade steel carrier with or without a lining is used (cf. DE 102 50 823 B4). Owing to the aggressive, less selective material-removing free lapping grain in the lapping slurry, the steel or high-grade steel carriers are subject to a high degree of wear.
The wear can be reduced somewhat if the thickness of the carriers is chosen to be significantly thinner than the final thickness of the semiconductor wafers. In this case, however, it is still at least 0.2-0.4 μm per lapping procedure with 90 μm target removal of material from the semiconductor wafers.
Owing to the continuous and considerable decrease in the thickness of the carriers, there is a continuous increase in the residual overhang of the semiconductor wafers upon reaching their target thickness over the residual thickness of the carriers. This leads to continuously changing machining conditions. The achievable flatness of the semiconductor wafers is considerably impaired as a result. The material abrasion from the carriers moreover leads to an additional contamination of the semiconductor wafers with trace metals. In order to ensure a reliable guidance of the semiconductor wafers in the receiving openings of the carriers, the overhang of the semiconductor wafer over the residual thickness of the carrier that is subject to wear is not permitted to exceed specific maximum values. For some profile shapes of the edges of the semiconductor wafers, the total wear of the carrier is not permitted to exceed as little as 10 μm, since otherwise the semiconductor wafers leave the receiving openings of the carriers during machining and fracture occurs. Therefore, the wear of the carrier is a major problem in the case of lapping, as well.
For applications in chemomechanical double-side polishing with colloidal silica in alkaline dispersion, carriers having a coating composed of plasma-deposited diamond-like carbon (DLC) have been proposed (US 2005/0202758 A1). The DLC coating effectively prevents contamination of the semiconductor wafers by metal. However, the production of the DLC coating is extremely complicated and expensive and makes the entire polishing process very expensive overall.
In particular when using diamond abrasive, the carrier materials known in the prior art are subject to very high wear. The material abrasion from the carrier adversely affects the cutting capacity (sharpness) of the working layers. This leads to an uneconomically short lifetime of the carriers and necessitates frequent unproductive redressing of the working layers.
Furthermore, a very high degree of wear was observed in all carriers composed of plastics known in the prior art with fiber reinforcement. This wear amounted to at least three up to a few tens of micrometers decrease in thickness of the carrier per operating procedure with 90 μm material removal from the semiconductor wafer. As a result, the carriers can only be utilized for a small number of procedures, which is uneconomical.
It has furthermore been shown that additional double-side coatings known in the prior art without fiber reinforcement, e.g. by means of lacquer or wear protection coatings composed of EP, EP/AC, PU/AC, etc., as disclosed e.g. in U.S. Pat. No. 5,882,245, are all subject to a very high degree of wear. In the case of EP and EP-based mixed coatings, moreover, they led to particularly rapid blunting of the working layer.
In particular, specific hard coatings proved to be totally unsuitable as coating for carriers for carrying out the PPG method. By way of example, a carrier coated with 3 μm DLC which can be utilized for a few hundred to well over a thousand operating procedures when used in double-side polishing (DSP) with colloidally disperse alkaline silica sol (chemomechanical polishing) was completely eroded down to the bare metal surface after just a few seconds when used in a PPG method. Ceramic or other hard substance coatings prove to be just as unsuitable.
Finally, it has been shown that some of the coating materials applied to the carrier core are exposed to very high (frictional) forces which lead to detachment of coatings produced by means of layer application methods known in the prior art.